DNA polymerases are naturally-occurring intracellular enzymes used by a cell for replicating DNA by reading one nucleic acid strand and manufacturing its complement. Enzymes having DNA polymerase activity catalyze the formation of a bond between the 3′ hydroxyl group at the growing end of a nucleic acid primer and the 5′ phosphate group of a newly added nucleotide triphosphate. Nucleotide triphosphates used for DNA synthesis are usually deoxyadenosine triphosphate (A), deoxythymidine triphosphate (T), deoxycytosine triphosphate (C) and deoxyguanosine triphosphate (G), but modified or altered versions of these nucleotides can also be used. The order in which the nucleotides are added is dictated by hydrogen-bond formation between A and T nucleotide bases and between G and C nucleotide bases.
Bacterial cells contain three types of DNA polymerases, termed polymerase I, II and III. DNA polymerase I is the most abundant polymerase and is generally responsible for certain types of DNA repair, including a repair-like reaction that permits the joining of Okazaki fragments during DNA replication. Polymerase I is essential for the repair of DNA damage induced by UV irradiation and radiomimetic drugs. DNA Polymerase II is thought to play a role in repairing DNA damage that induces the SOS response. In mutants that lack both polymerase I and III, polymerase II repairs UV-induced lesions. Polymerase I and II are monomeric polymerases while polymerase III is a multisubunit complex.
Enzymes having DNA polymerase activity are often used in vitro for a variety of biochemical applications including cDNA synthesis and DNA sequencing reactions. See Sambrook e al., Molecular Cloning: A Laboratory Manual (3rd ed. Cold Spring Harbor Laboratory Press, 2001, hereby incorporated by reference. DNA polymerases are also used for amplification of nucleic acids by methods such as the polymerase chain reaction (PCR) (Mullis et al., U.S. Pat. Nos. 4,683,195, 4,683,202, and 4,800,159, incorporated by reference) and RNA transcription-mediated amplification methods (e.g., Kacian et al., PCT Publication No. WO91/01384, incorporated by reference).
DNA amplification utilizes cycles of primer extension through the use of a DNA polymerase activity, followed by thermal denaturation of the resulting double-stranded nucleic acid in order to provide a new template for another round of primer annealing and extension. Because the high temperatures necessary for strand denaturation result in the irreversible inactivations of many DNA polymerases, the discovery and use of DNA polymerases able to remain active at temperatures above about 37° C. provides an advantage in cost and labor efficiency.
Thermostable DNA polymerases have been discovered in a number of thermophilic organisms including Thermus aquaticus, one strain of Thermus thermophilus, and certain species within the genera the Bacillus, Thermococcus, Sulfobus, and Pyrococcus. A full length thermostable DNA polymerase derived from Thermus aquaticus (Taq) has been described by Lawyer, et al., J. Biol. Chem. 264:6427–6437 (1989) and Gelfand et al, U.S. Pat. No. 5,079,352. The cloning and expression of truncated versions of that DNA polymerase are further described in Lawyer et al., in PCR Methods and Applications, 2:275–287 (1993), and Barnes, PCT Publication No. WO92/06188 (1992). Sullivan reports the cloning of a mutated version of the Taq DNA polymerase in EPO Publication No. 0482714A1 (1992). A DNA polymerase from Thermus thermophilus has also been cloned and expressed. Asakura et al., J. Ferment. Bioeng. (Japan), 74:265–269 (1993). However, the properties of the various DNA polymerases vary. Accordingly, new DNA polymerases are needed that have improved sequence discrimination, better salt tolerance, varying degrees of thermostability, improved tolerance for labeled or dideoxy nucleotides and other valuable properties.